An MRI (magnetic resonance imaging) instrument comprises a magnet portion, a transmitting-and-receiving portion for applying RF pulses or RF electromagnetic radiation to an object under examination placed in the magnetic field produced by the magnet portion and for detecting the NMR signal from the object, and a control-and-image processing portion which controls the operation of the transmitting-and-receiving portion and the operation of the magnet portion and processes the obtained data to image the object. The magnet portion consists of static magnetic field coils for generating a uniform and static magnetic field H.sub.0 and gradient magnetic field coils for setting up magnetic fields which are parallel with the static magnetic field H.sub.0 and have linear gradients in the directions of the x, y, z axes, respectively.
The control-and-image processing portion of the MRI instrument constructed as described above produces a pulse sequence by saturation recovery or repetitive inversion recovery method. Data is collected by making use of Fourier transform, and an image is reconstructed. At this time, data is collected from a square or rectangular region in a two-dimensional Fourier plane as shown in FIG. 9(a), by scanning it. When Fourier transform is employed, data concerning the nuclear spin density of the object is directly obtained by two-dimensional Fourier transform.
In FIG. 9(a), as the distance from the origin O increases, the spatial frequency increases. Let f be the highest spatial frequency in the horizontal and vertical directions. The highest spatial frequency in the diagonal direction is 2.sup.1/2 .multidot.f. Therefore, the resolution differs from direction to direction. Consequently, the obtained image becomes nonuniform in a peculiar way. The resolution in the diagonal direction is excessively high at the sacrifice of signal to noise ratio. In order to reconstruct an image free of these problems, a circular window function is applied to the data region as shown in FIG. 9(b). Specifically, the data contained in the hatched portion is discarded, and only the data contained in the circle is used.
In the conventional MRI instrument, a circular window function is applied to the data region to reconstruct an image. Therefore, even the data not used for the reconstruction is collected. This results in a useless operation during a scan.